Scoliosis is a spinal deformity that has an abnormal lateral curvature of the spine when viewed from a posterior perspective. The abnormal curvature of the spine is commonly associated with abnormal spinal rotation causing ribs to protrude posteriorly into what is commonly referred to as “rib hump”. The scoliosis is classified with infantile scoliosis and adolescent idiopathic scoliosis. The adolescent idiopathic scoliosis is the most prevalent type of scoliosis which develops during adolescence in an otherwise healthy patient and typically ceases at the onset of skeletal maturity. The cause of the disease is presently unknown.
Currently, surgical treatments of a spinal curvature deformity involve manipulation of the spinal column by attaching a correction device and then fusion of the spine. One such system, used primarily for scoliosis, is the Cotel-Dubousset system, as disclosed in U.S. Pat. No. 5,147,360 to Dubousset, which is understood to the use of rigidly attaching metal rods to the spine with plates and screws. The metal rods are then manipulated during the surgical procedure in an attempt to straighten the abnormal curvatures and reduce the rotation of the spinal column. The spine is then fused with a bone graft, typically requiring extensive discectomies, removal of spinous processes as the bone graft harvest, and injury to the spine itself to induce bleeding for improving the bone fusion. It is believed that the surgery is arduous, invasive, and has an array of potential complications including excessive blood loss. The discs above and below the fusion zone are in jeopardy of degeneration due to the increased biomechanical demands placed on them. Also, flat back syndrome could be problematic if normal lordosis and kyphosis are not restored. Recovery could be a lengthy and painful process. Even a successful procedure rarely results in a normal spinal curvature and the patient is left with an immobile spinal section.
A flaw in the conventional implants for correction of the spinal curvature deformity is that the implants are usually of a part of the load path of the spinal column. For example, it is understood that the Cotel-Dubousset system rigidly attaches stiff stainless steel rods to the spine. For a structure having two members placed in parallel, it relies primarily on the stiffest member for transmission of a load. Therefore, loads exerted on an instrumented spine are transferred through the implant instead of through the spine. Spinal loads could be large, and the geometry of the implants used is such that they may not support such large loads indefinitely. Fatigue failure of the implant occurs if fusion is delayed.
A further disadvantage of the conventional implants for correction of the spinal curvature deformity is the potential for neurologic damage. It has been shown that loads required for correction of the spinal curvature deformity during the surgery warrant concern for spinal cord trauma. For this reason, nerve functions are usually monitored during the surgery. Even after the surgery, the spinal loads could be large enough to cause nerve damages.
Additionally, viscoelastic properties of the spinal structures including the intervertebral discs, ligaments, nerves, muscles and other connective tissues have a time-dependent relationship between force and displacement: the stiffness of viscoelastic structures decreases with time under action of a sustained force. Stress-relaxation and creep are manifestations of viscoelasticity. The creep is gradual displacements under the action of the sustained force, while the stress-relaxation is gradual reductions of an internal force (resistance) under the action of an imposed displacement. It has been shown that dramatic stress-relaxation occurs within minutes of spinal curvature correction procedures. However, stiff instrumentation provides negligible additional correction even though resistive forces in the spine are decreasing.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.